Molecular imaging facilitates the early detection of disease, allows risk stratification, disease monitoring, longitudinal imaging and treatment follow up. A variety of imaging modalities have been developed, including positron electron tomography (PET), computed tomography (CT), and magnetic resonance imaging (MRI). The latter is gaining popularity because of its excellent soft tissue contrast, spatial resolution and penetration depth, and because the non-ionizing radiation is safer for repeated imaging sessions. However, MRI has a low sensitivity to contrast-enhancement agents, which provide important information about molecular features in vivo. Nanoparticles are ideal platforms for the development of better contrast-enhancement agents because they can carry large payloads, they can be modified with targeting ligands to confer molecular specificity and their structure enhances ionic relaxivity.
Several nanoparticle-based MRI contrast agents have been described, including nanoemulsions, dendrimers, silica and gold nanoparticles, and viral nanoparticles (VNPs). Bruckman et al., Nanotechnology. 2013; 24(46):462001. Nanoparticles increase the longitudinal relaxivity (positive contrast, R1) by reducing the molecular tumbling rate (τR) of chelated paramagnetic ions such as Gd following surface conjugation. Caravan et al., 2009; 4(2):89-100. In theory, free chelated Gd ions with a relaxivity of ˜5 mM−1s−1 can achieve relaxivities of up to 80 mM−1s−1 at 1.5 T, the common mode of MRI used in the clinic. This is based on the optimization of properties such as particle stiffness, bulk water accessibility and the chelating molecule, although experimentally it remains challenging to achieve such high values.
The inventors have focused the development of VNPs for medical applications because the manufacture of such proteinaceous nanoparticles in a variety of shapes and sizes is highly reproducible and scalable, and the particles themselves are amenable to functionalization using synthetic biology, genetic engineering and bioconjugation chemistry. Van Kan-Davelaar et al., British Journal of Pharmacology. 2014; 171(17):4001-4009. Several VNP-based MRI contrast agents have been described, including the icosahedral plant viruses Cowpea mosaic virus (CPMV) (Prasuhn et al., Chemical Communications. 2007(12):1269), Cowpea chlorotic mottle virus (CCMV) (Liepold et al., Magnetic Resonance in Medicine. 2007; 58(5):871-9), bacteriophages P22, MS2 and Qβ, and the plant virus Tobacco mosaic virus (TMV), which naturally occurs as rods but can also be produced as spheres. Bruckman et al., Journal of Materials Chemistry B. 2013; 1(10):1482A.
Few recent articles discuss the in vivo performance of these protein-based MRI contrast agents. Min et al., Biomacromolecules. 2013; 14(7):2332-9. For example, the inventors recently showed that TMV particles can be employed to image the molecular features of atherosclerotic plaques using a vascular cell adhesion molecule (VCAM-1)-targeted Gd(DOTA)-loaded probe. Bruckman et al., Nano Letters. 2014; 14(3):1551-8. The T1 relaxivity of this nanoparticle was ˜15 mM−1 s−1, yielding a per particle relaxivity of 35,000 mM−1 s−1 at 60 MHz, thus allowing the imaging of molecular features in vivo at submicromolar doses of Gd(DOTA). However, there remains a need for imaging agents with improved performance, such as increased sensitivity and decreased immunogenicity.